Growth and survival of Acidithiobacilli in Acidic, metal rich environments

Abstract: Acidithiobacilli are acidophilic microorganisms that play important roles in many natural processes such as acidification of the environment, influencing metal mobility, and impacting on global sulfur and iron cycles. Due to their distinct metabolic properties they can be applied in the industrial extraction of valuable metals. Acidithiobacilli thrive in an environment which is extremely acidic and usually low in organic carbon but highly polluted with metals. In the quest to gain insight into how these microorganisms can thrive in their extreme environment, relevant facets of metabolism, metal resistance, and pH homeostasis were exploredwith the focus on two model organisms,Acidithiobacillus caldus and Acidithiobacillus ferrooxidans. Understanding these fundamental aspects of an acidophilic lifestyle will help to eventually control detrimental effects on the environment due to acidification and metal pollution as well as improving metal extraction utilizing acidophilic microorganisms.Bioinformatics can give information about the genetic capacity of an organism. Likewise, ‘omics’ techniques, such as transcriptomics and proteomics to study gene transcription profiles and differentially expressed proteins canyield insights into general responses as well as giving clues regarding specific mechanisms for adaptation to life in extreme environments. This approach was used to investigate the sulfur metabolism ofAt. caldus which is an important sulfur oxidizer for industrial metal extraction. It was found that sulfur oxidation pathways were diverse within acidithiobacilli and a model of At. caldus sulfur oxidation was proposed. Furthermore, At. ferrooxidans anaerobic sulfur oxidation coupled to ferric iron reduction was studied which can be of importance for industrial processes. It was shown that anaerobic sulfur oxidation was, at least in part, indirectly coupled to ferric iron reduction via sulfide generation. Moreover, metal toxicity and resistance mechanisms in acidophiles are of major interest. Thus, zinc toxicity in three model organisms, At. caldus, Acidimicrobium ferrooxidans, and ‘Ferroplasma acidarmanus’, was explored. An important finding was that the speciation of metals and other chemical influences were of great importance for zinc toxicity in acidophiles. Additionally, the three organisms showed distinct responses to elevated zinc levels. Finally, the response of At. caldus to various suboptimal growth pH was evaluated to gain insights into pH homeostasis mechanisms. The results indicated that At. caldus used acid resistance mechanisms similar to those described for neutrophilic microorganisms. Analysis of fatty acid profiles demonstrated an active modulation of the cyctoplasmic membrane in response to proton concentration, likely resulting in a more rigid membrane at lower pH.